In general, when a vehicle body of an automobile, furniture, electric appliances and the like are to be coated, coating is performed by using a rotary atomizing head type coating machine which produces favorable paint coating efficiency and coating finish. This rotary atomizing head type coating machine is provided with a cylindrical housing having a motor accommodating portion, a cylindrical cover covering an outer peripheral side of the housing, an air motor rotating and driving a rotational shaft accommodated in the motor accommodating portion of the housing and supported by an air bearing by a turbine, a rotary atomizing head located on the front side of the housing and mounted on a distal end portion of the rotational shaft of the air motor and spraying a paint supplied while rotating together with the rotational shaft, a feed tube provided by being inserted through the rotational shaft and supplying the paint toward the rotary atomizing head, and a shaping air ring provided by surrounding an outer peripheral surface of the rotary atomizing head on the front end side of the housing and having an air ejection port for ejecting shaping air for shaping a spraying pattern of the paint sprayed from the rotary atomizing head.
In the housing of the rotary atomizing head type coating machine, a bearing air passage through which bearing air flows toward the air bearing of the air motor and a turbine air passage through which turbine air flows toward the turbine of the air motor are provided. Compressed air as the bearing air and the turbine air supplied to these air passages is discharged to the outside of the housing. Here, clean and fully dried dry air is used for the bearing air and the turbine air, and they are supplied at a predetermined pressure and flow rate.
On the other hand, the rotary atomizing head type coating machines include an electrostatic coating machine provided with a high-voltage generator for applying a high voltage to a paint supplied to the rotary atomizing head. In this case, paint particles charged to the high voltage can fly along an electric force line formed between the rotary atomizing head and the coating object to be coated and can coat the coating object to be coated efficiently.
In the recent rotary atomizing head type coating machine, a rotation speed of the rotary atomizing head, that is, the rotation speed of the turbine of the air motor has been raised so that a wide coating range can be efficiently coated by spraying a large quantity of paint. With this trend, the compressed air which becomes the turbine air to be supplied to the air motor has its pressure raised and the flow rate also increased.
As described above, in case the pressure of the turbine air is raised, when the high-pressure turbine air is ejected toward the turbine, the temperature of the high-pressure turbine air rapidly drops due to an action of adiabatic expansion. By means of this temperature drop, the air motor and the housing located on its outer peripheral side and the like are cooled.
Here, in a coating booth in which a coating work is performed, a temperature and humidity are controlled for favorable coating finish. For example, in a booth in which a body of an automobile is to be coated, the temperature is held at approximately 20 to 25° C., and the humidity is held at approximately 70 to 90%. Therefore, if the housing is cooled by the discharged air, condensation occurs on the surface of the cover covering the housing in the high-temperature high-humidity booth.
In the case of the electrostatic coating machine, in case condensation occurs on the surface of the cover, the high voltage to be applied to the paint leaks to the earth side, and electrostatic coating cannot be performed. Moreover, in case the cover is connected to the earth side by the condensation, the paint particles charged to the high voltage fly to the cover side and adheres to the cover surface, which becomes a factor for promoting drop of electric insulating property on the cover surface.
Moreover, not only in the case of the electrostatic coating machine but also in the case of non-electrostatic coating machine, if condensation on the cover surface progresses, the condensed moisture becomes drops of water. If the coating machine is operated in this state, the drops of water on the cover surface diffuse and adhere to the coated surface. In this case, even the drops of water in a mist state with small grain diameters remarkably deteriorate the coating quality in case they adhere to the coated surface regardless of the quantity of the drops of water.
Thus, the rotary atomizing head type coating machine for preventing condensation on the cover surface is used. This coating machine has a space portion so as to surround the periphery of the air motor and is configured such that heat insulating air is made to flow through this space portion. The heat insulating air is configured to be supplied to the space portion from an exclusively provided heat insulating air passage (See, Patent Document 1, for example).